Impact tests

Assessing the mechanical properties of adhesives and the joints strength requires specific tests. Indeed, since polymers are, in general, sensitive to the strain rate they undergo, different phenomena can be induced when a load is applied abruptly. Under such a condition, usually the adhesive tends to react to the deformation with higher stress and lesser ductility, which causes higher resisting loads but lower absorbed energy. This is why the common standard impact tests for adhesives, involving bonded blocks or strips, aim at measuring the energy required to break the bond. However, since this result is not sufficient to characterize an adhesive, other types of tests have been introduced to obtain a deeper insight. Different types of specimens (samples of bulk adhesive, lap joint, butt joint, double cantilever beam) and test rigs (pendulum, falling weight, hydraulic actuator) have been adopted in many works to measure the properties of interest. The fracture energy, typical of fracture mechanics, requires ad hoc tests and processing to be measured in dynamic conditions. The Split-Hopkinson pressure bar, a special apparatus conceived to test materials at high strain rate, has been also applied with success to adhesives and joints. Also test rigs based on the falling weight schemes have been widely used. A special case is that of conductive adhesives, which are getting into use to replace soldering in electronic packaging: their capability to withstand the impact that a device can undergo in use must be assessed. This chapter, mainly relying on the results available from the technical literature, presents a survey of various test methods, focusing on the related problems and achievements